Hydrocarbons and homologous series
A hydrocarbon contains only carbon and hydrogen. A homologous series shares the same general formula and functional group, a gradation in physical properties (boiling point rises with chain length) and similar chemical properties; each member differs by , so rises by 14. The two series: alkanes (), saturated (C–C singles), and alkenes (), unsaturated (C=C).
Functional groups and formula types
Three functional groups: hydroxyl –OH (alcohols), carboxyl –COOH (carboxylic acids) and the ester linkage –COO– (esters). Three formula forms: a molecular formula gives only atom counts (ethanol ); a structural (condensed) formula shows the sequence of groups (); a displayed formula draws every atom and bond, carbon 4 bonds, hydrogen 1, oxygen 2.
(Extended) Structural isomers
(Extended) Isomers share a molecular formula but have different structures. Butane () has two: n-butane, straight chain , and methylpropane, branched . Both have 4 C and 10 H but differ in shape, so branched methylpropane has weaker forces and a lower boiling point.
Drawn from real examiner reports.
Hydroxyl –OH vs carboxyl –COOH
An alcohol has a hydroxyl group –OH (one oxygen). A carboxylic acid has a carboxyl group –COOH — a C=O and an O–H on the same carbon (two oxygens), not just an "alcohol group". The ester linkage –COO– is also poorly recalled: it is not –CO– or –OO–. Hook: acid = C + OOH (two oxygens); alcohol = one OH; ester = the two joined.
–OH confused with –COOH; the ester linkage –COO– poorly known. (W22 P31 Q7(a)(i); W22 P41 Q6(c); W22 P42 Q6(d))
Homologous series needs all four features
"Same functional group" alone is not enough. All four are expected: the same general formula, the same (not "similar") functional group, a gradation in physical properties, and similar chemical properties. Not "same physical state" — members can be gas, liquid or solid. It is a family of carbon compounds, not a periodic group.
Definition given as 'similar functional group' or confused with Periodic Table groups. (W22 P31 Q7(c); S23 P31 Q7(b))
Molecular vs structural formula
Asked for a molecular formula, give only atom counts — no bonds or group notation. Writing for ethanoic acid, or for methanoic acid, gives a structural formula, losing the mark; the molecular formulae are and . No dashes or written groups.
Condensed structural formula (e.g. HCOOH) given when the molecular formula was asked. (W22 P42 Q6(c); W22 P41 Q6(a)(ii))
Carbon has 4 bonds in a displayed formula
In a displayed formula every bond is drawn as a line, and the valencies are fixed: each carbon has exactly 4 bonds, each hydrogen exactly 1, each oxygen exactly 2. Drawing a carbon with 5 bonds (pentavalent, often from adding an extra H) or a hydrogen sharing 2 bonds is penalised. After drawing, count the lines at every atom to check none is over- or under-bonded.
Displayed-formula drawings show pentavalent carbon or divalent hydrogen. (W22 P31 Q7(a)(i); W22 P41 Q6(c))
Saturated alkanes vs unsaturated alkenes
Alkanes are saturated (only C–C singles), ; cannot add hydrogen. Alkenes are unsaturated (a C=C bond), ; undergo addition across it. Mixing the formulae, e.g. writing for butane, is a common slip; butane is .
(Extended) Isomers vs isotopes
(Extended) These similar-sounding words are unrelated. Isomers are compounds with the same molecular formula but different structural formulae — about how atoms are arranged. Isotopes are atoms of the same element with the same proton number but different numbers of neutrons — about the nucleus. Using one for the other is a lost mark.
Count the oxygens to name the class
Count the oxygens in the functional group: no oxygen = hydrocarbon (C=C = alkene, else alkane); one oxygen as –OH = alcohol; two oxygens as –COOH = carboxylic acid; two oxygens bridging carbons as –COO– = ester. Sorts most 0620 structures.
Drawing a valid displayed formula
Draw the carbon chain; give each carbon 4 bonds, hydrogen 1, oxygen 2. Put the functional group in place; fill the rest with hydrogens. Check by counting lines. Show every atom and bond; is not displayed.
(Extended) Generating structural isomers
(Extended) For : draw the longest chain, then move a methyl branch along a one-shorter chain. For try two branches or ethyl. Rotate/reflect to drop duplicates; gives two isomers.
Hydrocarbon — a compound that contains only carbon and hydrogen atoms. All alkanes and alkenes are hydrocarbons.
Homologous series — a family of organic compounds with:
Each successive member differs from the previous by one unit ( increases by 14).
Functional group — the atom or group of atoms that gives a family of organic compounds its characteristic chemical properties.
Isomers (Extended) — compounds that have the same molecular formula but different structural formulae (different arrangements of atoms in the molecule).
Define hydrocarbon.
Propane is a member of the alkane homologous series. Its molecular formula is .
(a) Calculate the relative formula mass () of propane.
(: C = 12, H = 1)
(b) The next member of the alkane homologous series after propane is butane. State the molecular formula of butane and calculate its .
